LAGOS, Nigeria (CNN) -- The imprisoned leader of the Nigerian militant group MEND has accepted an unconditional amnesty offer from the government, his lawyer said Friday.

"The president has ordered his release, and we are hoping he will be released very soon," Wilson Ajuwaand said, referring to his client, Henry Okah. "We have briefed him on the amnesty and are now working out the details."

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CNN could not independently verify the lawyer's information, and Nigerian President Umaru Yar'Adua could not be reached for comment.

Yar'Adua made the amnesty offer to Okah in June.

Okah, who had been MEND's main arms smuggler, was arrested in September 2007 in the Angolan capital, Luanda. He was later extradited to Nigeria and has been in prison since then.

"We support Henry Okah's decision to accept any deal that will ensure his early release to attend to his failing health under the current circumstances," said Jomo Gbomo, a MEND spokesman. "Since he has no weapons to surrender, the deal should be a straightforward one."

An International Crisis Group report in April about the conflict between government forces and militants in the oil-rich Niger Delta said Okah is accused of treason, terrorism and kidnapping. His lawyers and relatives say he needs kidney treatment not available in Nigeria, the report said.

MEND has declared an "all-out war" on the government, demanding a fairer distribution of oil wealth in the Niger Delta. The group wants oil revenue reinvested in the region instead of enriching those the militants consider corrupt politicians. Many of MEND's attacks have been aimed at oil and gas installations in the region.

Okah's release had been a key demand of MEND before they accept any government amnesty offer.

After receiving news of Okah's acceptance of the government officer, MEND issued a statement.

"The recent appointment of Mr. Timi Alaibe as the special adviser to the president on Niger Delta-Nigeria-MEND-Amnesty affairs by the Nigerian government is a humble and welcome development that is in line with the two-pronged approach of our current campaign," the statement said.

"With this channel of communication finally opened, MEND will put together its ... team of frank negotiators to pass our demands and expectations to the president through Mr. Alaibe, a man we respect and can work with."